About every umg tutorial shows some stuff with bar.
Bars (and almost everything in umg) can also display warped (repeating) textures/materials.
All you need to do is calculate width of bar, then calculate percentage that single bullet (item) occupies and change bar at exactly those increments.
I guess what you could do is set the progress bar height to the height of the image and the width as the number of images * the image width. Ammo remaining / mag size should have the progress bar go up and down in the increments needed for that effect automatically since it is scaled to how many bullets fit in a mag.
Be sure to share your solution. I’d like to see if it is possible myself.